Value, generally.
GamersNexus did a great summary in their review, I think it broke down to:
All rounder, a Ryzen5 3600 makes more sense
Looking for the highest frame rates, an OC'd i5-10600K is the route to go (but is more expensive)
EDIT.
At the very least, watch the conclusion here:
View: https://youtu.be/csFwlKgZCzM?t=1014
To get the best performance out of a 10400, you'd want higher clocked memory, necessitating a Z series motherboard which is wasted with a locked CPU anyway.
So, for most people, the comparably priced R5 3600 just makes much more sense.